This invention relates to systems for, and methods of, operating in local area networks to provide for the transmission and reception of signals through unshielded twisted pairs of wires between a computer and a hub. The invention particularly relates to systems for, and methods of, using digital techniques for enhancing the recovery, and the quality of such recovery, of the digital signals passing through the unshielded twisted pairs to the computer so that the information represented by such digital signals can be restored at the computer.
Systems now exist for passing information between different computers in a local area network. The systems include a hub connected to computers located at spaced positions around the hub. The connections between the hub and each computer are generally through unshielded twisted pairs of wires. These wires are generally made from copper so that they have relatively large losses. This has limited the distance through which the signals can pass between the hub and each computer. The unshielded twisted pairs of wires have also limited the rate at which the signals can be transmitted. Until relatively recently, the distance between the hub and each computer has been limited to approximately one hundred (100 m.) and the rate of signal transmission has been limited to approximately 10 megabits per second (10 Mb/sec.).
The systems discussed in the previous paragraph and constituting the prior art have used analog techniques at the computer to recover the information represented by the digital signals. For example, the systems of the prior art have used analog equalizers to compensate for deteriorations in the characteristics of the digital signals as the digital signals pass through the unshielded twisted pairs of wires. These analog techniques have been satisfactory when the signals have passed through the unshielded twisted pairs of wires at a frequency of ten megabits per second (10 Mb/sec.)
The amount of information being transmitted through the unshielded twisted pairs of lines has been increasing at a relatively rapid rate. To provide for this increased transmission of information, the rate of transmission has been increased to one hundred megabits per second (100 Mb/sec.). The increased rate of signal transmission has prevented analog equalizers from operating effectively in restoring at the computer the signals transmitted from the hub.
Digital circuits have been considered for use in systems employing unshielded twisted pairs of wires and transmitting signals at one hundred megabits per second (100 Mb/sec) through distances as great as one hundred meters (100 m.). For example, digital adaptive equalization technology have been considered for such systems. However, digital systems have been rejected for several reasons. One reason has been that the systems considered have not provided significantly enhanced performance. Furthermore, the complexity of such systems has been quite high, particularly in relation to any enhanced performance obtained from such systems. The cost of such digital systems have also been considered to be excessive.
This invention provides a system for, and method of, receiving at a computer packets of digital signals transmitted from a hub displaced by a distance of as much as one hundred meters (100 m.) from the computer and for recovering the information represented by the digital signals in the packets. The system and method of this invention provide for such recovery whether the digital signals are transmitted through the wires at a frequency of ten megabits per second (10 Mb/sec.) or one hundred megabits per second (100 Mb/sec).
The system of this invention includes a digital adaptive equalizer for recovering the information represented by the digital signals in the packets. This equalizer is of an advanced design and includes feedback techniques to enhance the resolution provided by the equalizer in determining the amplitude level of each of the digital signals in each packet. The system and method of this invention are particularly adapted to operate with four (4) unshielded twisted pairs of wires, three (3) of the four (4) transmitting information whether the transmission is from the hub to the computer or from the computer to the hub. The system and method of this invention also include circuits and techniques for synchronizing the operation of the equalizer with the digital signals in the packets to enhance the recovery of the amplitudes of the digital signals by the equalizer.
In one embodiment of the invention, four (4) unshielded twisted pairs of wires connect a hub and a computer in an Ethernet system: one (1) pair of transmission only, another for reception only and the other two (2) both for transmission and reception. The signals in the wires are in packets each initially having timing signals defining a preamble and thereafter having digital signals representing information as by individual ones of three (3) amplitude levels.
The signals received at the computer are provided with an automatic gain control (AGC) and then with digital conversion at a particular rate. A control loop operative upon the digital conversions regulates the AGC gain at a particular value. An equalizer operative only during the occurrence of the digital signals representing information in each packet selects an individual one of the three (3) amplitude levels closest to the amplitude of each digital conversion at the time assumed to constitute the conversion peak.
The amplitudes of the timing signals in each preamble at the times assumed to constitute the peaks and zero crossings of such signals are multiplied. The rate of such digital conversions is adjusted in accordance with the polarity and magnitude of the multiplication product. The relative amplitudes of the successive equalizer values following each preamble are evaluated at the times assumed to be the peaks of the digital conversions. The rate of the digital conversions is adjusted in accordance with such evaluations, thereby further regulating the digital conversions at the particular rate. The equalizer thus operates on the information signals in each packet at the signal peaks.